The essential parts of the construction described above are known from Finnish patent FI-2820. The construction in question is, however, a so-called centrifugal screener. The object of this construction is to introduce pulp to a rotor having uniformly bent blades, the rotor accelerating the pulp into a rotary motion in a manner necessary in centrifugal screeners. Accelerating of pulp into a rotary motion requires much energy. Furthermore, rotating pulp passes the perforations in the screenplate with high velocity and, therefore, the pulp fibre velocity parallel with the screenplate must abruptly slow down and turn perpendicular to the screenplate in order to pass through the perforations of the screenplate.